Multicomponent sacrificial structure

ABSTRACT

A MEMS comprising a sacrificial structure, which comprises a faster etching portion and a slower etching portion, exhibits reduced damage to structural features when in forming a cavity in the MEMS by etching away the sacrificial structure. The differential etching rates mechanically decouple structural layers, thereby reducing stresses in the device during the etching process. Methods and systems are also provided.

BACKGROUND OF THE INVENTION

1. Technical Field

This application is generally related to microelectromechanical systems(MEMS), and more particularly, to MEMS with cavities and methods forforming the same.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF THE INVENTION

A MEMS comprising a sacrificial structure, which comprises a fasteretching portion and a slower etching portion, exhibits reduced damage tostructural features when in forming a cavity in the MEMS by etching awaythe sacrificial structure. The differential etching rates mechanicallydecouple structural layers, thereby reducing stresses in the deviceduring the etching process. Methods and systems are also provided.

Accordingly, some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a sacrificial structure formed over a firststructural layer; and a second structural layer formed over thesacrificial structure. The second structural layer comprises a pluralityof etchant access openings extending through the second structurallayer, the sacrificial structure comprises a first portion proximal tothe first structural layer and a second portion distal to the firststructural layer, one of the first portion and the second portion isselectively etchable in the presence the other of the first portion andthe second portion, and the sacrificial structure is selectivelyetchable in the presence of the first structural layer and the secondstructural layer.

In some embodiments, one of the first portion of the sacrificialstructure and the second portion of the sacrificial structure isetchable by a preselected etchant at a faster rate than the other of thefirst portion and the second portion. In some embodiments, the secondportion of the sacrificial structure is etchable by the preselectedetchant at a faster rate than the first portion of the sacrificialstructure.

In some embodiments, the sacrificial structure comprises a sacrificiallayer having a graded composition between the first portion of thesacrificial structure and the second portion of the sacrificialstructure.

In some embodiments, the first portion of the sacrificial structurecomprises a first sacrificial layer and the second portion of thesacrificial structure comprises a second sacrificial layer. In someembodiments, the first sacrificial layer and the second sacrificiallayer have different compressions. In some embodiments, the sacrificialstructure further comprises a third sacrificial layer, wherein at leastone of the first sacrificial layer and second sacrificial layer isetchable by a preselected etchant at a faster rate than the thirdsacrificial layer.

In some embodiments, the first portion of the sacrificial structurecomprises a plurality of sacrificial layers forming interface regionstherebetween, and the second portion comprises the interface regions. Insome embodiments, the sacrificial layers comprise substantially the samematerial formed under substantially the same conditions.

In some embodiments, the sacrificial structure comprises at least one ofW, Mo, Nb, Ta, Re, Cr, Ni, Al, Ga, In, Sn, Ti, Pb, Bi, Sb, B, Si, Ge,and combinations, alloys, or mixtures thereof. In some embodiments, thesacrificial structure comprises a photoresist.

In some embodiments, the preselected etchant comprises XeF₂. In someembodiments, an etching selectivity between the first portion of thesacrificial structure and the second portion of the sacrificialstructure is at least about 2.5:1 using the preselected etchant.

In some embodiments, the sacrificial structure consists of twosacrificial layers.

In some embodiments, the first structural layer comprises a dielectricmaterial. Some embodiments further comprise an electrode formed belowthe first structural layer. In some embodiments, the second structurallayer comprises a deformable layer.

Some embodiments further comprise: a movable reflective layer formedbetween the sacrificial structure and the second structural layer; aconnector coupling the second structural layer and the movablereflective layer; and a layer of a sacrificial material formed betweenthe second structural layer and the movable reflective layer. Someembodiments further comprise a support structure extending between thefirst structural layer and the second structural layer.

In some embodiments, the microelectromechanical systems device is aninterferometric modulator.

Some embodiments further comprise: a display; a processor that isconfigured to communicate with said display, said processor beingconfigured to process image data; and a memory device that is configuredto communicate with said processor.

Some embodiments further comprise a driver circuit configured to send atleast one signal to the display. Some embodiments further comprise acontroller configured to send at least a portion of the image data tothe driver circuit. Some embodiments further comprise an image sourcemodule configured to send said image data to said processor. In someembodiments, the image source module comprises at least one of areceiver, transceiver, and transmitter. Some embodiments furthercomprise an input device configured to receive input data and tocommunicate said input data to said processor.

Some embodiments provide a method of fabricating amicroelectromechanical systems device, the method comprising: formingover a first structural layer a sacrificial structure comprising a firstportion proximal to the first structural layer and a second portiondistal to the first structural layer, wherein the sacrificial structureis selectively etchable in the presence of the first structural layerand the second structural layer, and one of the first portion and thesecond portion is selectively etchable in the presence of the other ofthe first portion and the second portion; forming a second structurallayer over the sacrificial structure; and forming a plurality of etchantaccess openings extending through the second structural layer.

In some embodiments, one of the first portion and the second portion isetchable by a preselected etchant at a faster rate than the other. Someembodiments further comprise etching away one of the first portion andsecond portion using the preselected etchant. In some embodiments,etching away one of the first and second portions using the preselectedetchant comprises etching away one of the first and second portionsusing XeF₂.

In some embodiments, forming the sacrificial structure comprises forminga first sacrificial layer proximal to the first structural layer and asecond sacrificial layer distal to the first structural layer. In someembodiments, forming the sacrificial structure comprises forming asacrificial layer comprising a graded composition between the firstportion and the second portion. In some embodiments, forming thesacrificial structure further comprises forming a third sacrificiallayer, wherein a preselected etchant etches at least one of the firstsacrificial layer and second sacrificial layer faster than the thirdsacrificial layer.

Some embodiments provide a method of manufacturing amicroelectromechanical systems device comprising: forming a sacrificiallayer over a first layer; forming a second layer over the sacrificiallayer; selectively etching the sacrificial layer from between the firstlayer and the second layer to form at least one pillar extending betweenthe first layer and the second layer; and mechanically decoupling thesacrificial layer from at least one of the first layer and the secondlayer before etching away the at least one pillar.

In some embodiments, forming the sacrificial layer comprises forming alayer comprising at least one of germanium and molybdenum oxide. In someembodiments, forming the second layer comprises forming an aluminummovable reflective layer. In some embodiments, mechanically decouplingthe sacrificial layer comprises mechanically decoupling from the secondlayer.

Some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a first sacrificial layer contacting a firststructural layer; a second sacrificial layer formed over the firstsacrificial layer; and a second structural layer contacting the secondsacrificial layer. The first sacrificial layer and the secondsacrificial layer are selectively etchable in the presence of the firststructural layer and the second structural layer using a preselectedetchant, and one of the first sacrificial layer and second sacrificiallayer is etched by the preselected etchant at faster rate than theother.

Some embodiments further comprise a plurality of etchant access openingsextending through the second structural layer.

Some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a dielectric layer formed over a firstconductive layer; a sacrificial structure formed over the dielectriclayer; and a second conductive layer formed over the sacrificialstructure, wherein the sacrificial structure is selectively etchable inthe presence of the dielectric layer and the second conductive layerusing a preselected etchant, and the sacrificial structure comprises afaster etching portion and a slower etching portion with respect to thepreselected etchant.

In some embodiments, the sacrificial structure comprises a graded layerof the faster etching portion and the slower etching portion.

Some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a compositionally non-uniform sacrificialstructure formed over a first structural layer; and a second structurallayer formed over the sacrificial structure, wherein the secondstructural layer comprises a plurality of etchant access openingsextending through the second structural layer, the sacrificial structureis selectively etchable in the presence of the first structural layerand the second structural layer, and a preselected etchant etches thesacrificial structure non-uniformly.

Some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a first structural means for supporting themicroelectromechanical systems device; a sacrificial means for forming acavity in the microelectromechanical systems device; and a secondstructural means for actuating the microelectromechanical systemsdevice, wherein the second structural means comprises a plurality ofetchant access means for contacting the sacrificial means with anetchant means, and the sacrificial means comprises a faster etchingportion and a slower etching portion.

In some embodiments, the first structural means comprises a substrate.In some embodiments, the sacrificial means comprises a sacrificialstructure. In some embodiments, the second structural means comprises adeformable layer.

Some embodiments provide a method of manufacturing amicroelectromechanical systems device comprising: forming a sacrificialstructure over a first layer; forming a second layer over thesacrificial structure; and selectively etching away the sacrificialstructure substantially completely from between the first layer and thesecond layer using a preselected etchant, wherein the sacrificialstructure comprises a faster etching portion and a slower etchingportion with respect to the preselected etchant.

In some embodiments, forming a sacrificial structure comprises forming aplurality of sacrificial layers.

Some embodiments provide an apparatus comprising amicroelectromechanical systems device, wherein the micromechanicalsystems device comprises: a sacrificial structure formed over a firststructural layer; and a second structural layer formed over thesacrificial structure. The sacrificial structure comprises a firstportion and a second portion, one of the first portion and the secondportion has a faster intrinsic etching rate using a preselected etchant,the sacrificial structure is selectively etchable in the presence of thefirst structural layer and the second structural layer using thepreselected etchant, and an aspect ratio of a width or length tothickness of the sacrificial structure is at least about 50:1.

In some embodiments, the aspect ratio of the width and length tothickness of the sacrificial structure is at least about 50:1. In someembodiments, the aspect ratio of the width or length to thickness of thesacrificial structure is at least about 100:1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIGS. 8A-8E are cross sections of embodiments of unreleasedinterferometric modulators corresponding to the released interferometricmodulators illustrated in FIGS. 7A-7E.

FIG. 9A is a cross section of an embodiment of an unreleasedinterferometric modulator comprising a multicomponent sacrificialstructure. FIG. 9B is a top view of the unreleased interferometricmodulator illustrated in FIG. 9A.

FIG. 9C is a cross section of an embodiment of an unreleasedinterferometric modulator in which the sacrificial structure comprisestwo sacrificial layers.

FIG. 9D is a cross section of an embodiment of an unreleasedinterferometric modulator in which the sacrificial structure comprisesthree sacrificial layers.

FIGS. 9E-9G are cross sections of an embodiment of the interferometricmodulator illustrated in FIG. 9C at different stages of a release etch.

FIG. 9H is a cross section of an embodiment of an interferometricmodulator comprising a graded sacrificial structure at an intermediatestage of a release etch.

FIGS. 9I-9K are cross sections of an embodiment of an interferometricmodulator with a sacrificial structure comprising two similarsacrificial layers and intermediate stages in the etching thereof. FIG.9L is a cross section of an intermediate stage of etching of a similarinterferometric modulator with a single layer sacrificial structure.

FIG. 10 is a flowchart schematically illustrating an embodiment of amethod for manufacturing a MEMS using a multicomponent sacrificialstructure.

FIG. 11 a cross section of an embodiment of an interferometric modulatorcomprising a single component sacrificial structure at an intermediatestage of a release etch.

FIG. 12 is a cross section of an embodiment of an interferometricmodulator comprising a two-layer sacrificial structure.

FIG. 13 is through-substrate view of an array of interferometricmodulators manufactured using multicomponent sacrificial structuresafter a release etch.

FIG. 14 is a through-substrate view of an array of interferometricmodulators manufactured using single component sacrificial structures atan intermediate stage of a release etch.

FIG. 15 is a through-substrate view of an array of interferometricmodulators manufactured using single component sacrificial structures atan intermediate stage of a release etch.

FIG. 16 is a through-substrate view of an array of interferometricmodulators manufactured using single component weakly-adheringsacrificial structure after release.

FIG. 17 is a through-substrate view of an array of interferometricmodulators manufactured using two-component weakly-adhering sacrificialstructure after release.

FIG. 18 is a through-substrate view of an array of interferometricmodulators manufactured using another two-component weakly-adheringsacrificial structure after release.

FIG. 19 is an electron micrograph of the interface between the movablereflective layer and the two-component weakly-adhering sacrificialstructure used in the manufacture of the array illustrated in FIG. 18.

FIGS. 20A-20C are release radius maps for interferometric modulatorarrays comprising one-layer, two-layer, and three-layer sacrificialstructures, respectively.

FIGS. 21A and 21B illustrate results of release radius measurements ofinterferometric modulator arrays comprising one-layer, two-layer, andthree-layer sacrificial structures.

FIG. 22 illustrates relative etching rates for embodiments ofsingle-component and multicomponent sacrificial structures.

FIGS. 23A-23F are cross-sectional scanning electron micrographs ofpartially etched interferometric modulator arrays comprising one-layer,two-layer, and three-layer sacrificial structures.

DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

In some embodiments for manufacturing interferometric modulators andother MEMS devices, one or more cavities are created in the MEMS byetching away one or more sacrificial layers disposed between relativelymovable components of the completed MEMS, for example, the substrate andthe deformable layer. In this etching step, damage to the MEMS can occurbecause etching away a sacrificial layer permits motion between therelatively movable components even before the etching is completed. Asthe sacrificial layer is etched, the remaining portions form islandsand/or pillars extending between the relatively movable components. Inparticular, relative motion between the components causes stress atthese islands or pillars. If the stress becomes large enough, one of thecomponents will fail to relieve the stress. In some cases, the failureis to one or more components that are critical to the functioning of theMEMS. In some embodiments, damage may be prevented in the etchingprocess by employing a sacrificial structure between the relativelymovable components that mechanically decouples the relatively movablecomponents before the sacrificial structure is completely etched away.In some embodiments, the sacrificial structure is a non-uniformlyetchable sacrificial structure, for example, comprising at leastdifferentially etchable first portions and second portions.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in FIG. 1) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias) and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias) or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, the network interface27 can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, a driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, the input device 48includes a keypad, such as a QWERTY keyboard or a telephone keypad, abutton, a switch, a touch-sensitive screen, or a pressure- orheat-sensitive membrane. In one embodiment, the microphone 46 is aninput device for the exemplary display device 40. When the microphone 46is used to input data to the device, voice commands may be provided by auser for controlling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, the power supply 50 is arenewable energy source, a capacitor, or a solar cell including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections can take the form of continuous walls and/orindividual posts. For example, parallel rails can support crossing rowsof deformable layer 34 materials, thus defining columns of pixels intrenches and/or cavities between the rails. Additional support postswithin each cavity can serve to stiffen the deformable layer 34 andprevent sagging in the relaxed position.

The embodiment illustrated in FIG. 7D has support post plugs 42 uponwhich the deformable layer 34 rests. The movable reflective layer 14remains suspended over the gap, as in FIGS. 7A-7C, but the deformablelayer 34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIGS. 7A-7E, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties. Those skilledin the art will understand that in some embodiments, for example, theembodiments, illustrated in FIGS. 7A and 7B, the movable reflectivelayer 714 also acts as a deformable layer, in essence, an integratedmovable reflective layer and deformable layer.

Embodiments of MEMS devices comprising movable components or elementsare fabricated by a method in which a one or more sacrificial materialsis removed or etched from a precursor structure, thereby creating acavity or opening in the finished MEMS. Because such an etching stepreleases movable components from locked configurations in the precursorMEMS, such an etching step is referred to herein as a “release etch.”Accordingly, the precursor MEMS is also referred to as “unreleased”MEMS. Sacrificial structures comprising sacrificial materials serve asplaceholders in the manufacture of the MEMS, for example, as patternedlayers defining one or more voids, openings, and/or cavities in a MEMS.In particular, for electrostatic MEMS, sacrificial structures formedbetween stationary electrodes and movable electrodes occupy volumes thatform cavities in the finished device. For example, FIGS. 8A-8E,illustrate unreleased interferometric modulators that correspond to thereleased interferometric modulators illustrated in FIGS. 7A-7E,respectively. The unreleased interferometric modulators 800 comprise asubstrate 820 on which is formed an optical stack 816. A firstsacrificial structure 850 is formed on the optical stack 816. Areflective layer 814 is formed on the sacrificial structure 850 and onsupport structures 818 that extend through the sacrificial structure850. In the embodiments illustrated in FIGS. 8C-8E, a second sacrificialstructure 860 is formed over the reflective layer 814, which issuspended from a deformable layer 834. In FIGS. 8A and 8B, the layer 814represents both a deformable layer and a movable electrode or mirror. InFIGS. 8C-8E, the deformable layer 834 and movable electrode or mirror814 are separate structures.

In some embodiments, the release etch comprises exposing the unreleasedinterferometric modulators to one or more etchants that selectively etchthe first sacrificial structure 850 and, if present, the secondsacrificial structure 860, thereby forming the cavities in theinterferometric modulators illustrated in FIGS. 7A-7E, respectively,thereby releasing the reflective layer 814. In some embodiments, aplurality of suitable etchants is used to etch away the firstsacrificial structure 850 and/or the second sacrificial structure 860(if present). In some embodiments, the first and second sacrificialstructures 850, 860 are etched contemporaneously, while in others, theyare etched separately. Those skilled in the art will understand that theparticular etchant or etchants used in a particular process will dependon the identities of the sacrificial materials in the MEMS, theidentities of the structural materials, the structure of the MEMS, andthe like.

In some embodiments, the release etch is performed using a vapor phaseetchant that selectively etches both the first sacrificial structure 850and the second sacrificial structure 860 (if present). The vapor phaseetchant accesses the first sacrificial structure 850 and the secondsacrificial structure 860 through one or more etch holes (notillustrated) formed in the deformable layer 834, through gaps betweenstrips of the deformable layer 834, and/or from the sides of the device.In some preferred embodiments, the vapor phase etchant comprisesfluorine-based etchants, and in particular, vapor phase xenon difluoride(XeF₂). At ambient temperature, xenon difluoride is a solid with a vaporpressure of about 3.8 Torr (0.5 kPa at 25° C.). Vapor from xenondifluoride selectively etches certain sacrificial materials, that is,without forming a plasma.

Those skilled in the art will understand that the materials comprisingthe sacrificial structures are selected in conjunction with structuraland/or non-sacrificial materials of the device such that the sacrificialmaterial(s) are selectively etched over the structural materials. Inembodiments using XeF₂ as an etchant in the release etch, thesacrificial material may comprise at least one of silicon, germanium,titanium, vanadium, tantalum, molybdenum, tungsten, and mixtures,alloys, and combinations thereof; in some embodiments, molybdenum,tungsten, silicon, germanium, or silicon/molybdenum. In someembodiments, the sacrificial structure comprises an organic compound,for example, a polymer such as a photoresist. In some embodiments, asacrificial structure comprises a single layer. In other embodiments, asacrificial structure comprises a plurality of layers. Suitablestructural materials are known in the art. Where the etchant comprisesXeF₂, suitable structural materials resist etching by XeF₂, and include,for example, silica, alumina, oxides, nitrides, polymers, aluminum,nickel, chromium, and the like.

FIG. 9A illustrates a side cross-sectional view of an embodiment of anunreleased interferometric modulator 900 similar to the embodimentillustrated in FIG. 8D, which after release etching, provides a MEMSsimilar to the embodiment illustrated in FIG. 7D. Those skilled in theart will understand that certain features described with reference tothe illustrated embodiment are also useful in other embodiments ofinterferometric modulators, including embodiments illustrated in FIGS.7A-7C and 7E, as well as in other types of MEMS devices.

In the illustrated embodiment, the device 900 comprises a substrate 920on which is formed an optical stack 916 comprising a conductive layer916 a, a partially reflective layer 916 b, and a dielectric layer 916 c.The optical stack 916 represents a lower stationary electrode of theMEMS device. A support structure illustrated as comprising a pluralityof support post plugs 942 extends from the optical stack 916 andsupports a deformable layer 934. A movable reflective layer 914,representing a movable electrode for the MEMS device, is secured to thedeformable layer 934. A plurality of etchant access openings or etchholes 970 is formed in the deformable layer 934, as illustrated in FIG.9B, which is a top view of an array of devices 900. Those skilled in theart will understand that other arrangements of etch holes are used inother embodiments.

Between the optical stack 916 and the movable reflective layer 914 isformed a first sacrificial structure 950, and between the movablereflective layer 914 and deformable layer 934 is formed a secondsacrificial structure 960. Those skilled in the art will understand thatother embodiments, for example, some embodiments corresponding to FIGS.8A and 8B, comprise only a single sacrificial structure. In someembodiments, an aspect ratio between a length and/or width to a heightof the first sacrificial structure 950 is greater than about 50:1,greater than about 100:1, greater than about 500:1, or greater thanabout 1000:1. In some embodiments, the ratio between a length and/orwidth of the movable reflective layer 914 and the thickness of the firstsacrificial structure 950 is greater than about 50:1, greater than about100:1, greater than about 500:1, or greater than about 1000:1.

In the illustrated embodiment, the first sacrificial structure 950 iscompositionally non-uniform, for example, comprising at least a firstportion (or component) and a second portion (or component). Each of thefirst portion and second portion comprises one or more sacrificialmaterials such that one of the first portion and second portion has afaster intrinsic etching rate, and is thereby selectively and/ordifferentially etchable over the other. Those skilled in the art willunderstand that many materials are etchable by a given etchant underspecified conditions, but at rates too low for practical use in devicemanufacture. Accordingly, the term “etchable” as used herein refers tomaterials that a given etchant will etch at a useful rate for themanufacture of a device. In some embodiments, the first portion andsecond portion are different materials, each of which is etched by adifferent etchant. In some embodiments, the first portion and secondportion are different materials, both of which are etchable by a commonetchant. In some embodiments, the first and second portions comprise thesame material, but with different etch rates. For example, in someembodiments, the first and second portions are formed or deposited withdifferent levels of internal compression, density, and/or stress.Methods for fabricating materials with different levels of internalcompression, density, and/or stress are known in the art, for example,by controlling deposition parameters including power, bias, pressure,flow, combinations, and the like. In some embodiments, the first andsecond portions are differently doped. In some embodiments, at least oneof the first and second portion is modified, for example, by ionimplantation, passivation, or the like.

In some preferred embodiments, one of the first portion and secondportion is selectively etchable over the other using a single etchant,for example, XeF₂. As discussed above, sacrificial materials that areetchable by XeF₂ include silicon, germanium, titanium, vanadium,tantalum, molybdenum, tungsten, and mixtures, alloys, and combinationsthereof; in some embodiments, molybdenum, tungsten, silicon, germanium,or silicon/molybdenum. Examples of comparative bulk etching rates forcertain of these materials include W/Si, 2.5:1; Mo/Si, 6:1; Ti/Si, 85:1,Si/SiN, 1,000:1; Si/SiO₂, 10,000:1. In some embodiments, SiN and SiO₂are used as etch stops for XeF₂, and are more rapidly etched using otheretchants known in the art. Those skilled in the art will understand thatother combinations of materials are also useful in other embodiments.Where the etchant is XeF₂, embodiments include combinations of firstportions and second portions include, for example, W/Si, Mo/Si, Ti/Si.

In some embodiments of the first sacrificial structure 950, the firstportion comprises at least two regions of a material formed undersubstantially the same or similar conditions and the second portioncomprises an interface region between adjacent regions of the firstportion. For example, in some embodiments, the first portion comprisesthe bulk of first and second layers, and the second portion comprisesthe interface region between the first and second layers. In otherembodiments, the first portion comprises regions with another structure,for example, granules, rods, needles, or the like. Other embodimentscomprise a combination of these configurations. Those skilled in the artwill understand that the etch rate of the first portion regions will besubstantially similar or identical. Nevertheless, some embodimentsexhibit improved etching rates of the first sacrificial structure 950compared with similar, single component sacrificial structures, asdiscussed in greater detail below.

Suitable sacrificial materials for the first portions are similar to thesacrificial materials discussed above for sacrificial components withdifferent etching rates. In some embodiments, the sacrificialmaterial(s) are etchable using a fluorine-based etchant (e.g. XeF₂), forexample, comprising at least one of silicon, germanium, titanium,vanadium, tantalum, molybdenum, tungsten, and mixtures, alloys, andcombinations thereof. In some embodiments the sacrificial materialcomprises molybdenum, tungsten, silicon, germanium, and/orsilicon/molybdenum where the etchant comprises XeF₂.

It is believed that the interfacial layer that forms the second portioncomprises adjacent surface layers of the first portions, as well ascompounds formed on the surfaces of the adjacent surface layers, and anycontaminants formed and/or deposited thereon. The topmost layer of amaterial typically has a higher free energy than the bulk. Becausesurface atoms have neighbors below and to the sides, but none above, thelattice distorts at the surface. The distortions typically propagatedownward through a few layers of the material. These distortionsincrease the free energy of the top few layers of the material, therebyincreasing the reactivity of the atoms in these layers. The topmostatoms also have “dangling bonds” because these atoms have no neighborsabove them. Thus, the topmost atoms are extremely reactive, forming, forexample, oxides, hydroxides, nitrides, carbides, fluorides, hydrides,and the like, depending on the compounds in the ambient environment. Anew layer deposited over this surface layer will not be epitaxial unlessthe surface layer is scrupulously cleaned. Accordingly, the bottom-mostatoms of the new layer are deposited over a relatively “dirty” surface,and likely react with the underlying surface, as well. This interfaciallayer exhibits different etching characteristics than the bulk of thesacrificial layers.

In some embodiments, the first sacrificial structure 950 comprises agraded sacrificial layer, the composition of which changes from thefirst portion, to a mixture of the first and second portions, to thesecond portion. In some embodiments, the gradient between the firstportion and the second portion is generally vertical, that is, with ahigher concentration of one of the first and second portion at the top(e.g., proximal to the movable reflector 914) of the sacrificialstructure 950 and a higher concentration of the other of the first andsecond portion at the bottom (e.g., proximal to the optical stack 916)of the sacrificial structure 950. Preferably, the faster etching of thefirst and second portion is disposed proximal to the etch holes 970(FIG. 9B), which in the illustrated embodiment, are formed in thedeformable layer 934. Any suitable formation method is used infabricating the graded layer, for example, PVD-type, CVD-type, andALD-type processes, as well as combinations, and the like.

In some embodiments, for example, the embodiment illustrated in crosssection in FIG. 9C, the first portion of the first sacrificial structure950 comprises a first sacrificial layer 952 and the second portioncomprises a second sacrificial layer 954. In the illustrated embodiment,the first and second sacrificial layers 952, 954 are etchable by thesame etchant, preferably, a vapor phase etchant, for example, XeF₂. Insome embodiments, a faster etching sacrificial layer is disposedadjacent to a structural layer, for example, the movable reflector 914and/or optical stack 916, as will be discussed in greater detail below.In some embodiments, the faster-etching sacrificial layer is disposedproximal to the etch holes 970 (FIG. 9B). Accordingly, in someembodiments, the second sacrificial layer 954 is etched faster than thefirst sacrificial layer 952. In the release etch, the etchant contactsthe sacrificial structure 950 through etch holes 970 (FIG. 9B) in thedeformable layer 934, as discussed above.

The relative thicknesses of the sacrificial layers 952 and 954 areselected according to factors known in the art, for example, therelative etch rates, the overall etch rate of the first sacrificialstructure, the ease of forming each layer, deposition time for eachlayer, the thermal budget, residues left after etching, cost, and thelike. In some embodiments, the relative thickness of the layers 952 and954 are selected to provide early mechanical decoupling of thestructural layers, for example, by selection of the particular materialsfor each layer based on the relative differences in their etch rates. Insome embodiments, the relative thicknesses of the layers 952 and 954 arefrom about 1:100 to about 100:1, from about 10:90 to about 90:10, fromabout 20:80 to about 80:20, from about 40:60 to about 60:40, or about50:50. Those skilled in the art will understand that the overallthickness of the combined sacrificial layers 952 and 954 will depend onfactors including the color, if the device is an interferometricmodulator, for example, from about 50 nm to about 300 nm, for example,about 100 nm.

In other embodiments, the first sacrificial structure 950 comprisesother combinations of sacrificial layers, some of which are graded insome embodiments. For example, in some embodiments of the device 900illustrated in FIG. 9C, at least one of the first and second sacrificiallayers 952, 954 is a graded sacrificial layer. Other embodiments of thefirst sacrificial structure 950 comprise greater than two sacrificiallayers, one or more of which may be graded. For example, the embodimentof the first sacrificial structure 950 illustrated in FIG. 9D comprisesa first sacrificial layer 952, a second sacrificial layer 954, and athird sacrificial layer 956. As discussed above, in some embodiments,relatively faster etching sacrificial layers in the sacrificialstructure 950 are disposed adjacent to structural elements, for example,the movable reflector 914 and optical stack 916. Accordingly, in someembodiments, at least one of the first and third sacrificial layers 952,956 is etchable at a faster rate than the second sacrificial layer 954.

Each of the sacrificial layers, graded or non-graded, in the sacrificialstructure 950 is formed by any suitable formation method, for example,one or more of PVD-type, CVD-type, and/or ALD-type processes, as well asby spin coating, combinations, and the like.

In some embodiments comprising a second sacrificial structure, forexample, the second sacrificial structure 960 illustrated in FIG. 9C,the second sacrificial structure 960 has a non-uniform structure similarto that described above for the first sacrificial structure 950, forexample, comprising one or more graded and/or non-graded sacrificiallayers. In some embodiments, the second sacrificial structure 960comprises a single layer of a sacrificial material.

FIG. 10 is a flowchart illustrating an embodiment of a method 1000 forfabricating a MEMS with reference to the embodiments illustrated inFIGS. 9C-9G. Those skilled in the art will understand that the method1000 is applicable to the manufacture of MEMS of other designs as well.

In step 1010, a sacrificial structure is formed between a firststructural layer and a second structural layer. For example, in theembodiment illustrated in FIG. 9A, the first sacrificial structure 950is formed over a plurality of structural features, for example, thesubstrate 920 and the optical stack 916. As discussed above, in theillustrated embodiment, the optical stack 916 in turn comprises threelayers: a conductive layer 916 a, a partially reflective layer orabsorber 916 b, and a dielectric layer 916 c. The sacrificial structure950 is formed as discussed above, for example, by PVD-type, CVD-type,and/or ALD-type processes, by spin coating, by combinations thereof, andthe like. A second structural layer, for example, the deformable layer934, is formed over the sacrificial structure 950. The illustratedembodiment also comprises a movable reflective layer 914, which isanother structural feature formed over the sacrificial structure 950.

In optional step 1020, etch openings are formed in one of the structurallayers. For example, FIG. 9B illustrates etch openings 970 formed in thedeformable layer 934. The etch openings permit etchant access to atleast a portion of the sacrificial structure.

In step 1030, the sacrificial structure is etched away, therebymechanically decoupling the structural layers before the sacrificialstructure is completely etched away. In some embodiments, the etchantselectively etches the sacrificial structure over structural layers andfeatures under the etching conditions. In some embodiments, the sameetchant etches both the first portion and the second portion of thesacrificial structure, although at different rates. Other embodimentsuse different etchants for the first portion and the second portion. Forexample, in some of these embodiments, only one of the first or secondportion is etched in this step. In some embodiments, the etchant is avapor-phase etchant, for example, XeF₂.

FIG. 9E illustrates in cross section the device 900 of FIG. 9C afterpartial release etching. In the illustrated embodiment, the secondsacrificial structure has been completely etched away at this stage ofthe etching; in other embodiments, at least a portion of the secondsacrificial structure remains unetched. At the illustrated stage ofetching, portions of the second sacrificial layer 954 closest to theetch holes have been completely etched away. The remaining portions ofthe second sacrificial layer 954 that are relatively farther from theetch holes form the illustrated islands of sacrificial material.Similarly, portions of the first sacrificial layer 952 a exposed to theetchant are starting to etch. Those skilled in the art will understandthat the relative degree of etching between the first sacrificial layer952 and second sacrificial layer 954 will depend on their relativeetching rates by the selected etchant under the etching conditions.

It is believed that the etching occurs along an etch front thatpropagates horizontally and vertically along the interfaces betweendissimilar materials, for example, the second sacrificial layer 954 andthe bottom of the movable reflective layer 914. Behind the etch front,the bulk sacrificial layer 954 is etched, thereby forming the islandsillustrated in FIG. 9E. Those skilled in the art will understand that anetch rate of a material at an etch front does not necessarily correlateto the etch rate of the bulk material.

FIG. 9F illustrates the device 900 further along the etching process. Inthe illustrated embodiment, some unetched portions of the secondsacrificial layer form pillars 954 b extending between the deformablelayer 914 and the first sacrificial layer 952.

In some embodiments, the pillar 954 b becomes decoupled mechanicallyfrom at least one adjacent layer at this stage, for example, in theillustrated example, the movable reflective layer 914 or the firstsacrificial layer 952. Accordingly, structural layers above the secondsacrificial layer 954 are mechanically decoupled from structural layersbelow the second sacrificial layer. As the second sacrificial layer 954etches, stresses in the MEMS 900 between relatively movable componentsof the MEMS, in the illustrated example, components above the secondsacrificial layer 954 (e.g., the movable reflective layer 914, thedeformable layer 934) and the components below the second sacrificiallayer 954 (e.g., the first sacrificial layer 952, the optical stack 916,the substrate 920), are concentrated or focused at the pillar 954 b. Insome embodiments, a stress, for example, a shear and/or tensile stress,induces mechanical separation between the pillar 954 b and an adjacentlayer or structure, for example, at least one of the movable reflectivelayer 914 and the first sacrificial layer 952 in the illustratedembodiment.

For example, in some embodiments, the material of the pillar 954 doesnot adhere well to the material an adjacent layer, for example, of themovable reflective layer 914. The stress between these components causesa mechanical separation between the pillar 954 b and the movablereflective layer 914, for example, at their interface at the top 914 aof the pillar. In embodiments in which the movable reflective layer 914comprises aluminum and/or aluminum alloy, suitable weakly adhesivematerials for the second sacrificial layer 954 include germanium andmolybdenum oxide. Those skilled in the art will understand that othermaterials are useful in other embodiments, and that the particularmaterial will depend on the material to which the weakly adhesive layeris adjacent, the etching system, and the like. In some embodiments, thepillar 954 b and first sacrificial layer 952 become mechanicallydecoupled at this stage at the bottom 952 b of the pillar, that is, fromthe first sacrificial layer 952. In some embodiments, the firstsacrificial layer 952 adheres weakly to the dielectric layer 916 c,thereby facilitating mechanical decoupling between these layers. In someembodiments, each of the first and second sacrificial layers 952 and 954weakly adheres to an adjacent layer, for example, a structural and/orsacrificial layer. In other embodiments, the sacrificial structure 950comprises a single layer that weakly adheres to at least one adjacentlayer, for example, the movable reflective layer 914 and/or dielectriclayer 916 c.

Those skilled in the art will understand that a weakly adhering layeradheres sufficiently to an adjacent layer to permit fabrication of thedevice, but has sufficiently poor adhesion to decouple from the adjacentlayer during etching. As discussed above, the decoupling occurs as thecontact area between the weakly adhering layer and the adjacent isreduced as the weakly adhering layer is etched into islands and/orpillars. As the etching proceeds, the overall stress between the layersremains constant, thereby concentrating the stress on the islands and/orpillars. At some point, the stress overcomes the adhesion between thelayers and the layer decouple.

In other embodiments, the pillar 954 b does not become mechanicallydecoupled from an adjacent layer at the illustrated stage of etching.

FIG. 9G illustrates the device 900 further along in the etching process.At the illustrated stage of etching, the second sacrificial layer 954 issubstantially completely etched away, thereby mechanically decouplingthe layers above the second sacrificial layer (e.g., the movablereflective layer 914, the deformable layer 934) from the layers belowthe second sacrificial layer (e.g., the first sacrificial layer 952, theoptical stack 916, the substrate 920). In the illustrated embodiment,substantial portions of the first sacrificial layer 952 remain unetched,although in other embodiments, more of the first sacrificial layer 952is etched away at this stage.

In step 1040, the remainder of the sacrificial structure is etched away,using either the same etchant or a different etchant. In the deviceillustrated in FIG. 9G, etching away the remainder of the firstsacrificial layer 952 provides a device similar to that illustrated inFIG. 7D.

The method 1000 is also useful in manufacturing a MEMS using a device inwhich the sacrificial structure comprises a graded layer, for example,as illustrated in FIG. 9A. For example, in an embodiment in which agraded first sacrificial structure 950 comprises a faster etchingcomposition at the top and a slower etching composition at the bottom,in step 1030, the top of the sacrificial structure 950 etches relativelyrapidly horizontally and relatively slowly vertically, thereby providingthe sacrificial structure 950 illustrated in FIG. 9H. Completing theetching in step 1040 provides a device similar to that illustrated inFIG. 7D. Those skilled in the art will understand that the method 1000is also useful for release etching MEMS devices comprising a sacrificialstructure comprising combinations of graded and non-graded layers,and/or comprising greater than three sacrificial layers, as describedabove.

The method 1000 is also applicable to manufacturing a MEMS device fromembodiments of unetched MEMS devices comprising a first sacrificialstructure comprising a first portion of at least two regions of amaterial formed under substantially the same or similar conditions, anda second portion comprising an interface region between adjacent regionsof the first portion. In step 1010, a sacrificial structure is formedbetween structural layers. For example, an embodiment of the unetchedMEMS device 900 illustrated in FIG. 9I is similar to the deviceillustrated in FIG. 9C, and comprises a first sacrificial structure 950comprising a first sacrificial layer 952 and a second sacrificial layer954 corresponding to the first portion, and an interface or interfaciallayer 955 between the first 952 and second 954 sacrificial layers. Thoseskilled in the art will understand that some embodiments of thesacrificial structure 950 comprise more than two sacrificial layers. Thefirst 952 and second 954 sacrificial layers comprise substantially thesame sacrificial material formed under substantially the same conditionsin the illustrated embodiment. The first sacrificial structure 950 isdisposed between an optical stack 916 and a movable reflective layer914. The first 952 and second 954 sacrificial layers are formed usingany suitable method, for example, by sputtering, physical vapordeposition-type processes, chemical vapor deposition-type processes,atomic layer deposition-type processes, molecular beam epitaxy,combinations thereof, and the like. In some embodiments, the device iscleaned before and/or after depositing one or more of the sacrificiallayers.

Optional etching holes are formed in step 1020, which are not visible inthe section of the embodiment illustrated in FIG. 9I.

In step 1030 the first sacrificial structure 950 is etched using asuitable etchant. Suitable etchants and sacrificial materials arediscussed above. For example, in some embodiments, the etchant comprisesXeF₂ and the sacrificial materials comprise molybdenum. FIG. 9Jillustrates an intermediate structure in which the second sacrificiallayer 954 is etched through to the first sacrificial layer 952 and anetch front 958 propagates rapidly along an interface 955 between thefirst 952 and second 954 sacrificial layers, thereby forming a gap 959therebetween. Etching of the bulk sacrificial material in the first 952and second 954 sacrificial layers is slower. In FIG. 9K, etching of theinterface 955 is complete, and the resulting gap 959 mechanicallydecouples the first sacrificial layer 952 from the second sacrificiallayer 954.

The remaining portions of the first 952 and second 954 sacrificiallayers are etched away in step 1040 to provide the released MEMSillustrated in FIG. 7D. Some embodiments of this method exhibit at leastone of faster etching rates or reduced etchant usage for etching awaythe entire first sacrificial structure. It is believed that the improvedetching rates result from faster etching at the interface(s) or seamsbetween the first and second portions, thereby exposing larger surfacesof the first and second portions to the etchant than would be exposed inthe etching of a monolithic sacrificial structure. Although the precisemechanism for the faster etching at the interface 955 has not yet beendetermined, it is believed that surface strain of each layer as well ascompounds formed on the surfaces and contaminants trapped therein makesthe interface more prone to etching.

For example, in the embodiment illustrated in FIGS. 9J and 9K, the gap959 between the first 952 and second 954 sacrificial layers, formed byrapid etching of the interface 955 between the first 952 and second 954sacrificial layers, exposes horizontal surfaces 952 a and 954 b to theetchant, thereby increasing the overall etching rate. In contrast, FIG.9L illustrates an embodiment of a similar partially etched MEMS devicein which the first sacrificial structure comprises a single sacrificiallayer. In the illustrated embodiment, the first sacrificial structure950 is partially etched, forming openings 959 with etchant accessiblesurfaces 950 c, which have a smaller area than the surfaces 952 a and954 b. All other things being equal, the increased etchant accessiblearea in results in a faster etching rate of the first sacrificialstructure 950 in the embodiments illustrated in FIGS. 9I-9K comparedwith the embodiment illustrated in 9L.

Reduced etchant usage is believed to be related to increased etchingrate in some embodiments. In any etching process, some portion of theetchant will react at a relatively slow rate with one or more materialsother than the sacrificial material(s) of the sacrificial structure(s),for example, contaminants in the etching apparatus, structural materialsin the MEMS, and the like. By increasing the overall etching rate of thesacrificial material(s), contact time between the etchant and the sloweretching materials is reduced, thereby reducing the amount of etchantconsumed in this unproductive etching process.

FIG. 11 illustrates a cross section of a partially etched MEMS 1100similar to the device illustrated in FIG. 9F, except that the firstsacrificial structure 1150 comprises a single component instead of aplurality of components, for example, a layer comprising singlesacrificial material. In the illustrated embodiment, a pillar 1150 b ofthe unetched sacrificial material extends between the optical stack 1116and the movable reflective layer 1114. As discussed above, as theetching proceeds, relatively movable components of the MEMS become freeto move. This motion induces stress at those components of the MEMS thatremain relatively immobile, for example, the unetched portions of thesacrificial structure 1150. As material from the first sacrificialstructure 1150 is etched, the first sacrificial structure 1150 changesfrom a layer, to islands, to pillars 1150 b illustrated in FIG. 11. Asthe sacrificial structure 1150 gets smaller, the stresses get larger,because the relative motion of the movable components increases, andbecome more concentrated, in particular, at the top 1114 a of the pillarand at the bottom 1150 c of the pillar. Because the relative motion inthe device is vertical, the stress typically includes compressive ortensile components. In some cases, the stress is sufficient to causemechanical failure, which can be manifested as damage to one or more ofthe structural elements of the MEMS, for example, the optical stack 1116or the movable reflective layer 1114. In particular, damage to theoptical stack 1116 typically involves damage to the dielectric layer1116 c, for example, as a crack or break. A break in the dielectriclayer 1116 c, in turn, could permit contact between an etchant andstructures and/or layers below the dielectric layer 116 c, for example,the partial reflective layer 1116 b. Depending on the identities of theetchant and the material of the partial reflective layer 1116 b, in someembodiments, the partial reflective layer is etched and thereby damagedto at least some extent by the etchant after the break or damage to thedielectric layer 1116 c. Even in cases in which the partial reflectivelayer 1116 b is resistant to the etchant, because of the thinness of thepartial reflective layer 1116 b, any etching can be problematic to itsproper function.

In some embodiments of arrays of MEMS, the relative motion is greatestat a free edge of a deformable layer, and consequently, damage is mostlikely at the free edge. In some embodiments, an array or subarray ofMEMS shares a deformable layer. A free edge is an edge that is notshared, typically, at the edge of the array or subarray.

One technique for reducing the potential for damage created by theformation of pillars 1150 b in the etching of the sacrificial structure1150 is through modifying the etch holes, for example, increasing totalarea of the etch holes. The total area of the etch holes may beincreased by changing their dimensions, for example, changing theirshapes and/or increasing their sizes. The total area can also beincreased by increasing the number of etch holes. For example, someembodiments comprise a large number of etch holes with variabledimensions, which provide fast and controlled etching of the sacrificialstructure(s). Without being bound by any theory, it is believed thatincreasing the total area of the etch holes increases the etch rate ofthe sacrificial structure 1150. It is believed that in fast-etchingembodiments, pillars formed in the etching of the sacrificial structure1150 are etched away before they can damage the device. In someembodiments, however, increasing the areas of the etch holes reduces themechanical performance of the deformable layer 1134 in which they areformed. In some embodiments, a deformable layer having a large totalarea of etch holes increases the electrical resistance of the deformablelayer 1134. In some embodiments, a deformable layer having a large totalarea of etch holes negatively affects optical performance of an opticaldevice, for example, reducing the contrast ratio. Furthermore, largeand/or numerous etch holes reduce the fill factor of the MEMS in anarray in some embodiments.

In some embodiments, the etch holes are positioned such that anyetching-induced damage to the device is directed to non- or lesscritical portions of the device. In some embodiments, however, it is notpractical to prevent pillar formation in the etching reaction fromcertain areas in the device. In some embodiments, certain regions of thedeformable layer 1134 are not appropriate for forming etch holes.Providing non-critical areas can also reduce the fill factor in an arrayof MEMS.

In some embodiments, the sacrificial structure 1150 is made using a fastetching material, which, as discussed above, is believed to etch awaybefore appreciable damage is done to the device. Some embodiments offast-etching materials appear to form residues on etching that inducestiction in the finished device.

In contrast, embodiments of a MEMS comprising a sacrificial structure asdescribed exhibit some combination of fewer etch holes, smaller etchholes, mechanically more robust deformable layers, higher fill factors,and reduced stiction.

Example 1

An 5×6 array of unreleased interferometric modulators was manufacturedsimilar to the embodiment schematically illustrated in cross section inFIG. 12. The unreleased interferometric modulators 1200 comprise a 37 cm47 cm borosilicate glass substrate 1220 (about 0.7 mm thick), on whichis formed an optical stack 1216 comprising an indium tin oxide (ITO)layer (about 0.5 μm), a chromium layer (about 0.006 μm), and a silicondioxide layer (about 0.05 μm). A sacrificial structure 1250 comprising afirst sacrificial layer 1252 and a second sacrificial layer 1254 wasformed over the optical stack. An aluminum movable reflective layer 1214(about 0.03 μm) was formed over the sacrificial structure 1250. Etchholes 1270 are formed in the movable reflective layer 1214. Silicasupport posts 1218 were formed that extended between the substrate 1220and the movable reflective layer 1214. The lower, first sacrificiallayer 1252 comprised an about 50 nm thick layer of molybdenum, and theupper, second sacrificial layer 1254 comprised an about 50 nm thicklayer of germanium and/or silicon, both layers deposited by PVD. Afteretching with XeF₂ vapor (10 cycles of 120 s), all of the sacrificialstructure was etched away. FIG. 13 is a view through the substrate of aportion of the array 1300 showing black masks 1310 and etch holes 1320.The sacrificial material in the light-colored areas 1330 have beenetched away.

Example 2

An array of unreleased interferometric modulators similar to those inEXAMPLE 1 was manufactured with a sacrificial structure comprising asingle layer of molybdenum deposited by PVD. After etching with XeF₂vapor (10 cycles of 120 s), pillars of molybdenum remained, as well assome partially etched regions. FIG. 14 is a through-substrate view ofthe array 1400 showing black masks 1410 and etch holes 1420. Thelight-colored areas 1430 are fully etched. Darker areas 1440 arepartially etched. Pillars 1450 of molybdenum are observable around someof the black masks 1410.

Example 3

An array of unreleased interferometric modulators similar to those inEXAMPLE 1 was manufactured with a sacrificial structure comprising asingle layer of molybdenum deposited by PVD. After etching with XeF₂vapor (5 cycles of 120 s), the sacrificial layer was etched only underthe etch holes. FIG. 15 is a through-substrate view of the array 1500showing black masks 1510 and etch holes 1520 and the free edge 1570 ofthe deformable layer. The light-colored circular areas 1530 around theetch holes 1520 are etched. Dark areas 1560 are unetched.

Example 4

An array of interferometric modulators similar to those of EXAMPLE 1 wasmanufactured with a sacrificial structure comprising a single layer ofgermanium and an aluminum movable reflective layer. Etching with XeF₂vapor (5 cycles of 120 s) provided full release with no damage to theoptical stack. FIG. 16 is a through-substrate view of the etched arrayafter etching showing no defects in the optical stack. It is believedthat the germanium sacrificial structure adheres weakly to the movablereflective layer, and consequently, mechanically decouples these layers,thereby preventing damage to the interferometric modulator in theetching.

Example 5

An array of interferometric modulators similar to those of EXAMPLE 1 wasmanufactured with a sacrificial structure comprising a molybdenum firstsacrificial layer (about 50 nm thick) and a germanium second sacrificiallayer (about 50 nm thick). The movable reflective layer was aluminum.The array was etched with XeF₂ (5 cycles of 120 s), which provided fullrelease with no damage to the optical stack, as shown in thethrough-substrate view of the etched array in FIG. 17. It is believedthat poor adhesion between the germanium layer and the aluminum movablereflective layer prevents damage in the etching.

Example 6

An array of interferometric modulators similar to those of EXAMPLE 1 wasmanufactured with a sacrificial structure comprising a molybdenum firstsacrificial layer (about 50 nm) and a molybdenum oxide secondsacrificial layer (about 50 nm thick). The movable reflective layer wasaluminum. FIG. 18 is an scanning electron micrograph showing theinterface 1810 between the molybdenum oxide layer 1820 and the movablereflective layer 1830. The gap 1840 between the two layers indicatespoor adhesion between these materials. The array was etched with XeF₂ (5cycles of 120 s), which provided full release with no damage to theoptical stack, as shown in the through-substrate view of the etchedarray in FIG. 19. It is believed that poor adhesion between themolybdenum oxide layer and the aluminum movable reflective layerprevents damage in the etching.

Example 7

Unreleased interferometric modulators arrays similar to those in EXAMPLE1 were manufactured with one-layer, two-layer, and three-layersacrificial structures, where each layer was molybdenum deposited by PVDunder identical conditions. The underlying surface was cleaned beforedepositing each layer of molybdenum by PVD. Cleaning was performed bythermal degas, ion sputtering, or N₂O plasma. The thicknesses of eachsacrificial layer are provided in TABLE 1.

TABLE 1 1-Layer 2-Layer 3-Layer First layer 1,980 Å 1,100 Å   660 ÅSecond layer   880 Å   660 Å Third layer   660 Å Total 1,980 Å 1,980 Å1,980 Å

A “release radius” is the radius of an etched portion of the sacrificialstructure around an opening through which etchant contacts thesacrificial structure, for example, an etch hole. Accordingly, largerrelease radii correlate with faster etching. FIGS. 20A, 20B, and 20Cillustrate representative release radius contour maps for one array ofeach type after 14 XeF₂ etching cycles. Each hatching level is a 0.002mm radius contour. The map for the single-layer sacrificial structureillustrated in FIG. 20A shows faster etching at the center and sloweretching at the edges with a pronounced radial gradient, spanning sixdifferent release radius levels. The map for the two-layer sacrificialstructure illustrated in FIG. 20B exhibits more uniform etching over theentire array, with a slight bias toward the center. The map ofthree-layer sacrificial structure illustrated in FIG. 20C also exhibitsmore uniform etching compared with the single-layer, with a slight biastoward the edges. Etching rates increased as followings: three-layersacrificial structure>two-layer sacrificial structure>>one-layersacrificial structure.

Mean release radii values were then calculated for each array. FIG. 21Aillustrates average, minimum, and maximum values for several arrays ofeach type, again showing that the three-layer sacrificial structure isfaster than the two-layer, which is, in turn, much faster than theone-layer. The multilayer sacrificial structures also exhibited reducedsubstrate-to-substrate variation, which can improve process uniformity.

FIG. 21B illustrates the average, minimum, and maximum release radiivalues determined as discussed above, where the number of etching cyclesfor each type of sacrificial structure was selected to provide similaraverages. Similar release radii resulted after 14 etching cycles forone-layer sacrificial structures, 11 cycles for two-layer sacrificialstructures, and 10 cycles for three-layer sacrificial structures. Again,both multilayer sacrificial structures exhibited both faster etching andimproved uniformity compared with the single-layer sacrificialstructure.

The correlation between etch cycle and the pressure in the etching toolis illustrated in FIG. 22 for arrays comprising one-layer, two-layer,and three-layer sacrificial structures. The pressure in the etchingchamber increases during etching according to the following equation:

Mo(s)+3XeF₂(g)→MoF₆(g)+Xe(g)

Accordingly, etching is complete when the pressure drops to a stablevalue. Etching of the three-layer sacrificial structure was completeafter about 8 etching cycles, while the two-layer was complete afterabout 9 cycles, and the one-layer was complete after 12 cycles.

Arrays with one-layer, two-layer, and three-layer sacrificial structureswere partially released with two etching cycles. FIGS. 23A-23F are SEMimages of cross sections of an interferometric modulator from a cornerand the center of each array, as identified in TABLE 2.

TABLE 2 1-Layer 2-Layer 3-Layer Center FIG. 23A FIG. 23C FIG. 23E CornerFIG. 23B FIG. 23D FIG. 23F

In each image, an etch hole is formed in the left side of the upperlayer, and the etching proceeds to the right of the sacrificialstructure, which appears as a lighter colored layer in these images. Inthe etching of the single-layer sacrificial structure illustrated inFIGS. 23A and 23B, the etch front to the right of the etch hole isgenerally vertical. In contrast, in the multilayer sacrificialstructures illustrated in FIGS. 23C-23F, the etch front extendshorizontally, apparently along the interface between the sacrificiallayers. For the two-layer sacrificial structure illustrated in FIGS. 23Cand 23D, the etch front extends along the layer interface, tapering to apoint. The etch front of the three-layer sacrificial structure extendsalong both sacrificial layer interfaces, as illustrated in FIGS. 23E and23F, tapering to two points.

Those skilled in the art will understand that changes in the apparatusand manufacturing process described above are possible, for example,adding and/or removing components and/or steps, and/or changing theirorders. Moreover, the methods, structures, and systems described hereinare useful for fabricating other electronic devices, including othertypes of MEMS devices, for example, other types of optical modulators.

Moreover, while the above detailed description has shown, described, andpointed out novel features of the invention as applied to variousembodiments, it will be understood that various omissions,substitutions, and changes in the form and details of the device orprocess illustrated may be made by those skilled in the art withoutdeparting from the spirit of the invention. As will be recognized, thepresent invention may be embodied within a form that does not provideall of the features and benefits set forth herein, as some features maybe used or practiced separately from others.

What is claimed is:
 1. An apparatus comprising a microelectromechanicalsystems device, wherein the micromechanical systems device comprises: asacrificial structure formed over a first structural layer; and a secondstructural layer formed over the sacrificial structure, wherein thesecond structural layer comprises a plurality of etchant access openingsextending through the second structural layer, the sacrificial structurecomprises a first portion proximal to the first structural layer and asecond portion distal to the first structural layer, one of the firstportion and the second portion is selectively etchable in the presencethe other of the first portion and the second portion, and thesacrificial structure is selectively etchable in the presence of thefirst structural layer and the second structural layer.
 2. The apparatusof claim 1, wherein one of the first portion of the sacrificialstructure and the second portion of the sacrificial structure isetchable by a preselected etchant at a faster rate than the other of thefirst portion and the second portion.
 3. The apparatus of claim 2,wherein the second portion of the sacrificial structure is etchable bythe preselected etchant at a faster rate than the first portion of thesacrificial structure.
 4. The apparatus of claim 1, wherein thesacrificial structure comprises a sacrificial layer having a gradedcomposition between the first portion of the sacrificial structure andthe second portion of the sacrificial structure.
 5. The apparatus ofclaim 1, wherein the first portion of the sacrificial structurecomprises a first sacrificial layer and the second portion of thesacrificial structure comprises a second sacrificial layer.
 6. Theapparatus of claim 5, wherein the first sacrificial layer and the secondsacrificial layer have different compressions.
 7. The apparatus of claim5, wherein the sacrificial structure further comprises a thirdsacrificial layer, wherein at least one of the first sacrificial layerand second sacrificial layer is etchable by a preselected etchant at afaster rate than the third sacrificial layer.
 8. The apparatus of claim1, wherein the first portion of the sacrificial structure comprises aplurality of sacrificial layers forming interface regions therebetween,and the second portion comprises the interface regions.
 9. The apparatusof claim 8, wherein the sacrificial layers comprise substantially thesame material formed under substantially the same conditions.
 10. Theapparatus of claim 1, wherein the sacrificial structure comprises atleast one of W, Mo, Nb, Ta, Re, Cr, Ni, Al, Ga, In, Sn, Tl, Pb, Bi, Sb,B, Si, Ge, and combinations, alloys, or mixtures thereof.
 11. Theapparatus of claim 1, wherein the sacrificial structure comprises aphotoresist.
 12. The apparatus of claim 1, wherein the preselectedetchant comprises XeF₂.
 13. The apparatus of claim 1, wherein an etchingselectivity between the first portion of the sacrificial structure andthe second portion of the sacrificial structure is at least about 2.5:1using the preselected etchant.
 14. The apparatus of claim 4, wherein thesacrificial structure consists of two sacrificial layers.
 15. Theapparatus of claim 1, wherein the first structural layer comprises adielectric material.
 16. The apparatus of claim 15, further comprisingan electrode formed below the first structural layer.
 17. The apparatusof claim 1, wherein the second structural layer comprises a deformablelayer.
 18. The apparatus of claim 1, further comprising: a movablereflective layer formed between the sacrificial structure and the secondstructural layer; a connector coupling the second structural layer andthe movable reflective layer; and a layer of a sacrificial materialformed between the second structural layer and the movable reflectivelayer.
 19. The apparatus of claim 1, further comprising a supportstructure extending between the first structural layer and the secondstructural layer.
 20. The apparatus of claim 1, wherein themicroelectromechanical systems device is an interferometric modulator.21. The apparatus of claim 1, further comprising: a display; a processorthat is configured to communicate with said display, said processorbeing configured to process image data; and a memory device that isconfigured to communicate with said processor.
 22. The apparatus ofclaim 21, further comprising a driver circuit configured to send atleast one signal to the display.
 23. The apparatus of claim 22, furthercomprising a controller configured to send at least a portion of theimage data to the driver circuit.
 24. The apparatus of claim 21, furthercomprising an image source module configured to send said image data tosaid processor.
 25. The apparatus of claim 24, wherein the image sourcemodule comprises at least one of a receiver, transceiver, andtransmitter.
 26. The apparatus of claim 21, further comprising an inputdevice configured to receive input data and to communicate said inputdata to said processor.
 27. A method of fabricating amicroelectromechanical systems device, the method comprising: formingover a first structural layer a sacrificial structure comprising a firstportion proximal to the first structural layer and a second portiondistal to the first structural layer, wherein the sacrificial structureis selectively etchable in the presence of the first structural layerand the second structural layer, and one of the first portion and thesecond portion is selectively etchable in the presence of the other ofthe first portion and the second portion; forming a second structurallayer over the sacrificial structure; and forming a plurality of etchantaccess openings extending through the second structural layer.
 28. Themethod of claim 27, wherein one of the first portion and the secondportion is etchable by a preselected etchant at a faster rate than theother.
 29. The method of claim 28, further comprising etching away oneof the first portion and second portion using the preselected etchant.30. The method of claim 29, wherein etching away one of the first andsecond portions using the preselected etchant comprises etching away oneof the first and second portions using XeF₂.
 31. The method of claim 27,wherein forming the sacrificial structure comprises forming a firstsacrificial layer proximal to the first structural layer and a secondsacrificial layer distal to the first structural layer.
 32. The methodof claim 27, wherein forming the sacrificial structure comprises forminga sacrificial layer comprising a graded composition between the firstportion and the second portion.
 33. The method of claim 31, whereinforming the sacrificial structure further comprises forming a thirdsacrificial layer, wherein a preselected etchant etches at least one ofthe first sacrificial layer and second sacrificial layer faster than thethird sacrificial layer.
 34. A method of manufacturing amicroelectromechanical systems device comprising: forming a sacrificiallayer over a first layer; forming a second layer over the sacrificiallayer; selectively etching the sacrificial layer from between the firstlayer and the second layer to form at least one pillar extending betweenthe first layer and the second layer; and mechanically decoupling thesacrificial layer from at least one of the first layer and the secondlayer before etching away the at least one pillar.
 35. The method ofclaim 34, wherein forming the sacrificial layer comprises forming alayer comprising at least one of germanium and molybdenum oxide.
 36. Themethod of claim 35, wherein forming the second layer comprises formingan aluminum movable reflective layer.
 37. The method of claim 34,wherein mechanically decoupling the sacrificial layer comprisesmechanically decoupling from the second layer.
 38. An apparatuscomprising a microelectromechanical systems device, wherein themicromechanical systems device comprises: a first sacrificial layercontacting a first structural layer; a second sacrificial layer formedover the first sacrificial layer; and a second structural layercontacting the second sacrificial layer, wherein the first sacrificiallayer and the second sacrificial layer are selectively etchable in thepresence of the first structural layer and the second structural layerusing a preselected etchant, and one of the first sacrificial layer andsecond sacrificial layer is etched by the preselected etchant at fasterrate than the other.
 39. The apparatus of claim 38, further comprising aplurality of etchant access openings extending through the secondstructural layer.
 40. An apparatus comprising a microelectromechanicalsystems device, wherein the micromechanical systems device comprises: adielectric layer formed over a first conductive layer; a sacrificialstructure formed over the dielectric layer; and a second conductivelayer formed over the sacrificial structure, wherein the sacrificialstructure is selectively etchable in the presence of the dielectriclayer and the second conductive layer using a preselected etchant, andthe sacrificial structure comprises a faster etching portion and aslower etching portion with respect to the preselected etchant.
 41. Theapparatus of claim 40, wherein the sacrificial structure comprises agraded layer of the faster etching portion and the slower etchingportion.
 42. An apparatus comprising a microelectromechanical systemsdevice, wherein the micromechanical systems device comprises: acompositionally non-uniform sacrificial structure formed over a firststructural layer; and a second structural layer formed over thesacrificial structure, wherein the second structural layer comprises aplurality of etchant access openings extending through the secondstructural layer, the sacrificial structure is selectively etchable inthe presence of the first structural layer and the second structurallayer, and a preselected etchant etches the sacrificial structurenon-uniformly.
 43. An apparatus comprising a microelectromechanicalsystems device, wherein the micromechanical systems device comprises: afirst structural means for supporting the microelectromechanical systemsdevice; a sacrificial means for forming a cavity in themicroelectromechanical systems device; and a second structural means foractuating the microelectromechanical systems device, wherein the secondstructural means comprises a plurality of etchant access means forcontacting the sacrificial means with an etchant means, and thesacrificial means comprises a faster etching portion and a sloweretching portion.
 44. The apparatus of claim 43, wherein the firststructural means comprises a substrate.
 45. The apparatus of claim 43,wherein the sacrificial means comprises a sacrificial structure.
 46. Theapparatus of claim 43, wherein the second structural means comprises adeformable layer.
 47. A method of manufacturing a microelectromechanicalsystems device comprising: forming a sacrificial structure over a firstlayer; forming a second layer over the sacrificial structure; andselectively etching away the sacrificial structure substantiallycompletely from between the first layer and the second layer using apreselected etchant, wherein the sacrificial structure comprises afaster etching portion and a slower etching portion with respect to thepreselected etchant.
 48. The method of claim 47, wherein forming asacrificial structure comprises forming a plurality of sacrificiallayers.
 49. An apparatus comprising a microelectromechanical systemsdevice, wherein the micromechanical systems device comprises: asacrificial structure formed over a first structural layer; and a secondstructural layer formed over the sacrificial structure, wherein thesacrificial structure comprises a first portion and a second portion,one of the first portion and the second portion has a faster intrinsicetching rate using a preselected etchant, the sacrificial structure isselectively etchable in the presence of the first structural layer andthe second structural layer using the preselected etchant, and an aspectratio of a width or length to thickness of the sacrificial structure isat least about 50:1.
 50. The apparatus of claim 49, wherein the aspectratio of the width and length to thickness of the sacrificial structureis at least about 50:1.
 51. The apparatus of claim 49, wherein theaspect ratio of the width or length to thickness of the sacrificialstructure is at least about 100:1.